Variable resistance sensor with common reference leg

ABSTRACT

A resistive bridge sensor circuit that includes a first resistive bridge circuit having a first variable resistance resistor and a second resistive bridge circuit having a second variable resistance resistor. The first and second resistive bridge circuits share at least a portion of a common reference leg that sets a resistance of a first and second variable resistors. The common reference leg or a portion of the common reference leg is alternately switchably connected to one of the first and second resistive bridge circuits.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C. 119(e) to U.S.provisional application serial No. 60/436, 207, filed Dec. 23, 2002, andU.S. provisional application serial No. 60/397,139, filed Jul. 19, 2002,each entitled VARIABLE RESISTANCE SENSOR WITH COMMON REFERENCE LEG,which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

[0002] The present invention is directed to a resistive sensor, and moreparticularly to a mass flow sensor having separate upstream anddownstream circuits that are capable of detecting the mass flow rate ofa fluid and which share at least a portion of a common reference leg.

DESCRIPTION OF RELATED ART

[0003] Mass flow sensors are used in a wide variety of applications tomeasure the mass flow rate of a gas or other fluid. One application inwhich a mass flow sensor may be used is a mass flow controller. In aconventional mass flow controller, the mass flow rate of a fluid flowingin a main fluid flow path is regulated or controlled based upon a massflow rate of a portion of the fluid that is diverted into a typicallysmaller conduit forming a part of the mass flow sensor. Assuming laminarflow in both the main flow path and the conduit of the sensor, the massflow rate of the fluid flowing in the main flow path can be determined(and regulated or controlled) based upon the mass flow rate of the fluidflowing through the conduit of the sensor.

[0004] Two different types of mass flow sensors have traditionally beenused, constant current mass flow sensors, and constant temperature massflow sensors. An example of a constant current mass flow sensor isillustrated in FIG. 1. In FIG. 1, a fluid flows in a sensor pipe orconduit in the direction of the arrow X. Heating resistors or “coils” R₁and R₂ having a large thermal coefficient of resistance are disposedabout the sensor conduit on downstream and upstream portions of thesensor conduit, respectively, and are provided with a constant current Ifrom a constant current source 901. As a result of the constant currentI flowing through the coils R₁ and R₂, voltages V₁ and V₂ are developedacross the coils. The difference between voltages V₁ and V₂ (V₁−V₂) istaken out of a differential amplifier 902, with the output of theamplifier 902 being proportional to the flow rate of the fluid throughthe sensor conduit.

[0005] At a zero flow rate, the circuit of FIG. 1 is configured so thatthe resistance value (and thus, the temperature) of coil R₁ is equal tothe resistance value (and temperature) of coil R₂, and the output of theamplifier 902 is zero. As fluid flows in the sensor conduit, heat thatis generated by coil R₂ and imparted to the fluid is carried towards R₁.As a result of this fluid flow, the temperature of coil R₂ decreases andthat of coil R₁ increases. As the voltage dropped across each of theseresistors is proportional to their temperature, voltage V₁ increaseswith an increased rate of fluid flow and voltage V₂ decreases, with thedifference in voltage being proportional to the mass rate of flow of thefluid through the sensor conduit.

[0006] An advantage of a constant current mass flow sensor is that itcan operate over a wide range of temperatures, is relatively simple inconstruction, and is responsive to changes in the ambient temperature ofthe fluid entering the sensor conduit. As the ambient temperature of thefluid entering the sensor conduit changes, so does the resistance ofeach of the coils R₁ and R₂. However, it takes a relatively long timefor the temperature (and thus, the resistance) of the coils R₁ and R₂ tostabilize in response to a change in the rate of flow of the fluid.

[0007] The other type of mass flow sensor that is frequently used is aconstant temperature mass flow sensor, examples of which are illustratedin FIGS. 2-4. As shown in the constant temperature mass flow sensor ofFIG. 2, heating resistors or “coils” R_(1A) and R_(1B) are respectivelydisposed about the downstream and upstream portions of a sensor conduitthrough which a fluid flows in the direction of the arrow X. As in theconstant current mass flow sensor of FIG. 1, each of the downstream andupstream coils R_(1A) and R₁B has a large thermal coefficient ofresistance. The resistance (and thus the temperature) of each of thecoils R_(1A), R_(1B) is fixed by separate and independent circuits tothe same predetermined value that is governed by the value of resistorsR_(2A), R_(3A), R_(4A), and R_(2B), R_(3B), R_(4B), respectively.Control circuitry is provided to maintain each of the coils R_(1A),R_(1B) at the same predetermined value of resistance (and thus,temperature) independently of the rate of fluid flow through the sensorconduit.

[0008] In the absence of fluid flow, the circuit of FIG. 2 is configuredso that the resistance (and temperature) of each of the downstream andupstream coils R_(1A) and R_(1B) is set to the same predetermined valueand the output of the circuit is zero. As fluid flows in the sensorconduit, heat from the upstream coil R_(1B) is carried towards R_(1A).As a result, less energy is required to maintain the downstream coilR_(1A) at the fixed temperature than is required to maintain theupstream coil R_(1B) at that same fixed temperature. The difference inenergy required to maintain each of the coils R_(1A), R_(1B) at thepredetermined temperature is measured and is proportional to the massflow rate of fluid flowing through the sensor conduit.

[0009] The constant temperature mass flow sensor described with respectFIG. 2 is also relatively easy to construct. In addition, the circuit ofFIG. 2 stabilizes more quickly in response to changes in the mass flowrate of the fluid entering the sensor conduit than the constant currentmass flow sensor described with respect to FIG. 1. However, because eachof the coils R_(1A) and R_(1B) is set and maintained at a predeterminedtemperature independently of the ambient temperature of the fluidflowing into the sensor conduit, a problem arises when the ambienttemperature of the fluid flowing into the sensor conduit increases. Inparticular, when the ambient temperature of the fluid flowing in thesensor conduit approaches the predetermined temperature that ismaintained by the upstream and downstream coils, the circuit loses itsability to discern differences in the flow rate of the fluid, and whenthe ambient temperature of the fluid increases beyond this predeterminedtemperature, the sensor is rendered inoperable.

[0010] To overcome these disadvantages, a number of alternative constanttemperature mass flow sensors have been provided. For example, thecircuit of FIG. 3 provides a constant temperature mass flow sensor thatis capable of responding to changes in the ambient temperature of a gasor fluid, at least to a certain degree. Once again, R_(1B) and R_(2B)are downstream and upstream temperature sensing coils with a largetemperature coefficient of resistance. However, rather than maintainingthe temperature of the coils at a predetermined constant value as in thecircuit of FIG. 2, the circuit of FIG. 3 maintains the temperature ofthe sensor coils R_(1B), R_(2B) at a temperature that is above theambient temperature of the fluid flowing into the sensor conduit. Thisis achieved by the insertion of an additional coil R_(3B), R_(4B) havinga coefficient of resistance similar to that of the sensor coils R_(1B),R_(2B) in each of the downstream and upstream circuits. As the ambienttemperature of the fluid changes, the series addition of coil resistanceR_(3B), R_(4B) to the temperature setting resistors R_(5B), R_(6B)results in raising the temperature to which the upstream and downstreamresistance coils are maintained above the ambient temperature of thefluid flowing into the sensor conduit. As in the circuit of FIG. 2, thedifference in energy supplied by each of the downstream and upstreamcircuits to maintain the temperature of the coils R_(1B), R_(2B) at thesame temperature is proportional to the mass flow rate of the fluidthrough the sensor conduit.

[0011] As should be appreciated by those skilled in the art, for thecircuit of FIG. 3 to operate properly, it is critical that the valuesand thermal characteristics of each element in the downstream circuitmatch that of the corresponding element in the upstream circuit. Thus,the resistance of the downstream and upstream coils R_(1B), R_(2B) musthave the same value, and the same thermal coefficient of resistance. Inaddition, resistor R_(3B) must have the same value and the same (ideallylarge) thermal coefficient of resistance as resistor R_(4B), resistorR_(5B) must have the same value and same (ideally zero) thermalcoefficient of resistance as resistor R_(6B), resistor R_(7B) must havethe same value and same (ideally zero) thermal coefficient of resistanceas resistor R_(10B), resistor R_(9B) must have the same value and same(ideally zero) thermal coefficient of resistance as resistor R_(8B), andamplifiers 911 and 912 must have the same operating and temperaturecharacteristics.

[0012] Despite the addition of resistors R_(3B) and R_(4B), a problemwith the circuit of FIG. 3 is that as the ambient temperature of thefluid flowing into the sensor conduit rises, the sensor becomes lessaccurate because the proportional difference between the temperature ofthe upstream and downstream coils relative to the temperature of theambient fluid becomes smaller. Further, there is a problem due to driftin that the calibration of the sensor at one temperature does notnecessarily allow its use at other ambient temperatures without somesort of compensation circuit.

[0013] To solve some of the aforementioned problems, U.S. Pat. No.5,401,912 proposes a constant temperature rise (above ambient) mass flowsensor, an example of which is shown in FIG. 4. The circuit of FIG. 4acts to maintain upstream and downstream sensor coils R₂, R₁ at apredetermined value above the ambient temperature of the fluid flowinginto the sensor conduit. The circuit of FIG. 4 is identical to thecircuit of FIG. 2, except that the fixed value resistors R_(3A) andR_(3B) of FIG. 2, which have an essentially zero thermal coefficient ofresistance, are replaced with resistors R₅ and R₆, respectively, havinga large and specific valued thermal coefficient of resistance. As aresult of these changes, the circuit of FIG. 4 purportedly maintains aconstant temperature rise over the ambient temperature of the fluidflowing into the sensor conduit. Such a mass flow sensor as is shown inFIG. 4 is therefore termed a constant temperature difference (overambient) or a constant temperature rise (over ambient) mass flow sensor.

[0014] Each of the aforementioned constant temperature mass flow ratesensors utilizes separate and independent upstream and downstreamcircuits to set the temperature of the upstream and downstream coils toa particular value, or to a particular value over the ambienttemperature of the fluid flowing into the sensor conduit. A disadvantageof each of these circuits is that they require a close matching ofcorresponding circuit elements (i.e., resistors, coils, and amplifiers)in the upstream and downstream circuits.

SUMMARY OF THE INVENTION

[0015] According to one aspect of the present invention, a sensor isprovided that includes a first resistive bridge circuit having a firstvariable resistance resistor and a second resistive bridge circuithaving a second variable resistance resistor. According to oneembodiment, the first and second resistive bridge circuits share acommon reference leg that sets a resistance of the first and secondvariable resistors. The common reference leg is alternately switchablyconnected to one of the first and second resistive bridge circuits.According to another embodiment, the first and second resistive bridgecircuits share only a portion of the reference leg which sets aresistance of the first and second variable resistors. In thisembodiment, the portion of the common reference leg that sets theresistance of the first and second variable resistors is alternatelyswitchably connected to one of the first and second resistive bridgecircuits.

[0016] According to one embodiment, the sensor comprises a firstcircuit, a second circuit, a voltage divider, and at least one switch.The first circuit includes a first resistor having a first resistancethat varies in response to a change in a physical property. The secondcircuit includes a second resistor having a second resistance thatvaries in response to the change in the physical property. The at leastone switch has a first state and a second state. The first state of theat least one switch electrically connects the voltage divider to thefirst circuit to set the resistance of the first resistor, and thesecond state of the at least one switch electrically connects thevoltage divider to the second circuit to set the resistance of thesecond resistor.

[0017] According to another embodiment, the sensor comprises a firstamplifier and a second amplifier each having a first input, a secondinput, and an output, a first resistor and a second resistor, and avoltage divider. The first resistor is electrically connected in serieswith a first variable resistor between the output of the first amplifierand a reference terminal, with the first resistor being electricallyconnected between the first input of the first amplifier and the outputof the first amplifier, and the first variable resistor beingelectrically connected between the first resistor and the referenceterminal. The second resistor is electrically connected in series with asecond variable resistor between the output of the second amplifier andthe reference terminal, with the second resistor being electricallyconnected between the first input of the second amplifier and the outputof the second amplifier, and the second variable resistor beingelectrically connected between the second resistor and the referenceterminal. The voltage divider has an input that is switchably connectedto one of the output of the first amplifier and the output of the secondamplifier, and an output that is switchably connected to one of thesecond input of the first amplifier and the second input of the secondamplifier. The output of the voltage divider sets a resistance of thefirst variable resistor when the input of the voltage divider isconnected to the output of the first amplifier and the output of thevoltage divider is connected to the second input of the first amplifier,and sets a resistance of the second variable resistor when the input ofthe voltage divider is connected to the output of the second amplifierand the output of the voltage divider is connected to the second inputof the second amplifier.

[0018] According to another aspect of the present invention, a methodfor use with a pair of bridge circuits is provided: Each bridge circuithas a sensor leg that includes a fixed resistor and a variable resistorand a reference leg that sets a resistance of the variable resistor. Themethod comprises an act of sharing at least a portion of the referenceleg between the first and second circuits to match the resistance of thevariable resistors.

[0019] According to another aspect of the present invention, a flowsensor is provided to measure a flow rate of a fluid. The flow sensorcomprises a first variable resistor, a second variable resistor disposeddownstream of the first variable resistor when a flow of the fluid is ina first direction, a first circuit, electrically coupled to the firstvariable resistor, to provide a first signal indicative of powerprovided to the first variable resistor, a second circuit, electricallycoupled to the second variable resistor, to provide a second signalindicative of power provided to the second variable resistor, and athird circuit, to receive the first and second signals and provide anoutput signal indicative of a difference between the first and secondsignals. The range of the output signal when the flow of fluid is in thefirst direction is symmetric to the range of the output signal when theflow of the fluid is in a second direction that is opposite to the firstdirection.

[0020] According to a further aspect of the present invention, a flowsensor to measure a flow rate of a fluid is provided. The flow sensorcomprises a first variable resistor, a second variable resistor, a firstcircuit, a second circuit, a third circuit, and a power supply circuit.The first circuit is electrically coupled to the first variable resistorto provide a first signal indicative of power provided to the firstvariable resistor. The second circuit is electrically coupled to thesecond variable resistor to provide a second signal indicative of powerprovided to the second variable resistor. The third circuit receives thefirst and second signals and provides an output signal indicative of adifference between the first and second signals. The power supplycircuit is electrically connected to at least one of the first andsecond circuits to provide a variable amount of power to at least one ofthe first and second circuits dependent upon the flow rate of the fluid.

[0021] According to yet a further aspect of the present invention, amethod of detecting a high flow condition in a flow sensor is provided.The method comprises acts of determining an expected zero flow signal ata current operating temperature of the flow sensor, determining athreshold based upon the expected zero flow signal, determining anactual flow signal measured by the flow sensor at the current operatingtemperature of the flow sensor, comparing the actual flow signalmeasured by the flow sensor to the threshold, and determining that thehigh flow condition exists when the actual flow signal exceeds thethreshold.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Illustrative, non-limiting embodiments of the present inventionare described by way of example with reference to the accompanyingdrawings, in which:

[0023]FIG. 1 is a constant current mass flow sensor according to theprior art;

[0024]FIG. 2 is a constant temperature mass flow sensor according to theprior art;

[0025]FIG. 3 is a constant temperature mass flow sensor that is capableof responding to changes in an ambient temperature of a fluid accordingto the prior art;

[0026]FIG. 4 is another constant temperature mass flow sensor that iscapable of responding to changes in an ambient temperature of a fluidaccording to the prior art;

[0027]FIG. 5 is a schematic overview of a constant temperature mass flowsensor according to one embodiment of the present invention thatincludes upstream and downstream resistive bridge circuits that share acommon reference leg;

[0028]FIG. 6 is a schematic overview of a constant temperature mass flowsensor according to another embodiment of the present invention in whichthe common reference leg includes a programmable voltage divider;

[0029]FIG. 7 is a detailed schematic diagram illustrating one exemplaryimplementation of a constant temperature mass flow sensor in accordancewith FIG. 6;

[0030]FIG. 8 is a detailed schematic diagram illustrating anotherexemplary implementation of a constant temperature mass flow sensor inwhich the common reference leg includes a programmable voltage divider;

[0031]FIG. 9A is a schematic overview of a constant temperature massflow sensor according to another embodiment of the present invention inwhich the upstream and downstream resistive bridge circuits share only aportion of the common reference leg;

[0032]FIG. 9B is a detailed schematic diagram illustrating one exemplaryimplementation of a constant temperature mass flow sensor in accordancewith FIG. 9A;

[0033]FIG. 10 is a detailed schematic diagram of a flow sensor amplifiercircuit that may be used with any of the embodiments of FIGS. 5-9 toprovide a flow signal having a range that is symmetric independent ofthe direction of flow through the mass flow sensor; and

[0034]FIG. 11 is a detailed schematic diagram of a variable output powersupply which may be used with any of the embodiments of FIGS. 5-9.

DETAILED DESCRIPTION

[0035] Embodiments of the present invention will be understood morecompletely through the following detailed description which should beread in conjunction with the attached drawings.

[0036]FIGS. 5, 6, 7 and 8 illustrate a number of different mass flowsensors according to various embodiments of the present invention. Ineach of FIGS. 5, 6, 7, and 8, the reference designator R_(U) representsthe upstream coil or resistor, and reference designator R_(D) representsthe downstream coil or resistor. As in the sensor circuits of the priorart, coils R_(U) and R_(D) are disposed at spaced apart positions abouta sensor conduit (not shown) through which a fluid flows. As definedherein, the term fluid includes any material or combination of materialsin a solid, liquid, or gaseous state.

[0037] Each of coils R_(U) and R_(D) has a large and substantiallyidentical thermal coefficient of resistance, such that the resistance ofeach coils R_(U), R_(D) varies with temperature. Although the upstreamand downstream coils R_(U) and R_(D) are referred to herein as “coils,”it should be appreciated that the present invention is not so limited.For example, the upstream and downstream coils need not be coils thatare wound about the exterior of sensor conduit, but may be formed fromheat sensitive resistors that are simply affixed to the exterior of thesensor conduit. Such heat sensitive resistors need not have a coiledshape, as they may have a serpentine or ribbon shape instead. Moreover,the upstream and downstream coils need not be disposed on the exteriorof the sensor conduit, as for certain fluids, such as air, the coils maybe disposed within the sensor conduit. In addition, although embodimentsof the present invention are described in terms of a mass flow sensor,the present invention is not so limited, as aspects of the presentinvention may be used in other applications in which variations in theresistance of a leg of a resistive bridge circuit is indicative of achange in a property that varies with resistance.

[0038] Although mass flow sensors according to embodiments of thepresent invention use separate upstream and downstream resistive bridgecircuits to set the temperature and resistance of upstream anddownstream coils to an identical value, the upstream and downstreamcircuits share at least a portion of a common reference leg. As aresult, embodiments of the present invention do not require the closematching of component values and characteristics that is required in theseparate upstream and downstream circuits of FIGS. 2-4. Further, inthose embodiments in which the shared and common portion of thereference leg includes a programmable voltage divider, the range ofresistance to which the upstream and downstream coils are set can bevaried to accommodate a wide range of ambient temperatures.

[0039]FIG. 5 illustrates a simplified schematic diagram of a mass flowsensor according to one embodiment of the present invention. The sensorcircuit includes an upstream resistive bridge circuit 10 and adownstream resistive bridge circuit 20 that are used to set theresistance and, thus, the temperature of the upstream coil R_(U) and thedownstream coil R_(D), respectively. The upstream and downstreamcircuits 10, 20 share a common reference leg 30 that in the embodimentdepicted in FIG. 5 includes resistors R₁ and R₂ connected in series.

[0040] The upstream circuit 10 includes a first amplifier U₁ and aseries connection of a first resistor R_(UR) and a first variableresistor R_(U) (the upstream coil) electrically connected between anoutput of the first amplifier U₁ and a reference terminal. Electricallyconnected between the output of the first amplifier U₁ and the inverting(−) input of the first amplifier U₁ is a series connection of a resistorR_(U2) and a capacitor C_(U2). Electrically connected between themid-point of the first resistor R_(UR) and the first variable resistorR_(U) and the inverting input of the first amplifier U₁ is anotherresistor R_(U1). A relatively large valued capacitor C_(U1) iselectrically connected between the non-inverting (+) input of the firstamplifier U₁ and the reference terminal. The capacitor C_(U1) maintainsa voltage present at the non-inverting input of the first amplifier U₁when the common reference leg 30 is electrically connected to thedownstream circuit 20.

[0041] The downstream circuit 20 is similar to the upstream circuit 10.The downstream circuit 20 includes a second amplifier U₂ and a seriesconnection of a second resistor R_(DR) in series with a second variableresistor R_(D) that is connected between the output of the secondamplifier U₂ and the reference terminal. The inverting (−) input of thesecond amplifier U₂ is electrically connected to a mid-point of theseries connection of the second resistor R_(DR) and the second variableresistor R_(D) through a resistor R_(D1), and a resistor R_(D2) inseries with a capacitor C_(D2) is electrically connected between theoutput of the second amplifier U₂ and the inverting input of the secondamplifier U₂. The non-inverting (+) input of the second amplifier U₂ iselectrically connected a large valued capacitor C_(D1) that is connectedto the reference terminal.

[0042] As shown in FIG. 5, the circuit further includes a commonreference leg 30 that includes a first resistor R₁ electricallyconnected in series with a second resistor R₂ that is switchablyconnected to each of the upstream and downstream circuits 10, 20. Thecommon reference leg R₁, R₂ sets the value of resistance to which theupstream coil R_(U) and the downstream coil R_(D) are set and acts as avoltage divider. Switches 1 _(A) and 2 _(A) are connected between aninput of the voltage divider formed by the series connection R₁ and R₂and the output of the first and second amplifiers U₁, U₂, respectively.Switches 1 _(B) and 2 _(B) are each respectively connected to the outputof the voltage divider and the non-inverting input of the first andsecond amplifiers U₁ and U₂, respectively. Switches 1 _(A) and 1 _(B)and switches 2 _(A) and 2 _(B) work in tandem such that switches 1 _(A)and 1 _(B) and switches 2 _(A) and 2 _(B) are both open or closed at thesame time.

[0043] During operation, switches 1 _(A), 1 _(B), and switches 2 _(A), 2_(B) are alternately opened and closed to connect the common referenceleg 30 to one of the upstream and downstream circuits 10, 20. During thetime interval in which the common reference leg is not connected to theupstream circuit (i.e., when switches 1 _(A), 1 _(B) are open) thecapacitor C_(U1) maintains the voltage level at the non-inverting inputof the first amplifier U₁. Similarly, during the time interval in whichthe common reference leg is not connected to the downstream circuit(i.e., when switches 2 _(A) and 2 _(B) are open), the capacitor C_(D1)maintains the voltage level at the non-inverting input terminal of thesecond amplifier U₂.

[0044] In operation, the sensor circuit behaves as two constanttemperature driver circuits sharing a common reference leg. Switches 1and 2 are switched rapidly to connect the reference leg 30 (R₁ and R₂)to the upstream and downstream circuits alternately. C_(U1) and C_(D1)hold the sampled reference feedback when the corresponding switches areopen. The first amplifier U₁ servos such that R_(U)/R_(UR)=R₂/R₁. Thesecond amplifier U₂ servos so that R_(D)/R_(DR) =R₂/R₁. Other amplifiers(not shown in FIG. 5) pick off the upstream and downstream voltagelevels V_(U) and V_(D) between the series connection of the firstresistor R_(UR) and the first variable resistor R_(U) and the seriesconnection of the second resistor R_(DR) and the second variableresistor R_(D). The voltage levels V_(U) and V_(D) can then be used toprovide a signal that is indicative of the flow rate of fluid throughthe conduit in which, or about which, the upstream and downstream coilsR_(U), R_(D) are disposed. For example, in one embodiment, the ratio ofV_(U)−V_(D)/V_(D) provides the signal that is indicative of the flowrate of fluid, although other comparisons of the voltage levels V_(U)and V_(D) may alternatively be used as discussed in detail furtherbelow. The remaining components illustrated in FIG. 5, namely R_(U1),R_(U2), C_(U2), R_(D1), R_(D2), and C_(D2), are used to stabilize thefirst and second amplifiers U₁ and U₂.

[0045] It should be appreciated that because the upstream and downstreamcircuits 10, 20 share a common reference leg 30 that includes theidentical components, the sensor circuit of FIG. 5 dispenses with theneed closely match these components. That is, because both the upstreamand downstream circuits share the same components of the reference leg,those components are necessarily matched. Thus, in the circuit of FIG.5, although the ratio of R_(UR) to R_(DR) should be stable and theresistance of R_(UR) and R_(DR) preferably have the same value, it isnot required that they be identically matched. It should also beappreciated that in the schematic of FIG. 5, switches 1 and 2 should beclosed only after the voltage divider formed by the common reference leghas had an opportunity to stabilize. Although the sensor circuitdepicted in FIG. 5 may have some switching noise, this switching noisemay be controlled appropriately by switching the switches at anappropriate frequency, for example, at a frequency at or below theNyquist rate of the A to D converters (not shown) that receive thevoltage levels V_(U) and V_(D) (where such A to D converters are used),as known to those skilled in the art.

[0046] It should further be appreciated that while the simplifiedschematic drawing of FIG. 5 functionally depicts the operation of thesensor circuit, the circuit can be modified in a variety of ways. Forexample, high power amplifiers may be needed to provide an appropriateamount of current to the upstream and downstream coils. Alternatively,the output of the first and second amplifiers U₁ and U₂ may beelectrically connected to a large output transistor to provide anappropriate amount of current. Moreover, the present invention is notlimited to the use of four switches 1 _(A), 1 _(B), and switches 2 _(A),2 _(B), as fewer switches may be used. It should further be appreciatedthat in various implementations, the common reference leg 30 formed byR₁ and R₂ may be replaced with a programmable voltage divider. Thus,with appropriate control of the programmable voltage divider, aprogrammable temperature rise sensor driver may be provided. Anembodiment of a flow sensor that includes a programmable voltage divideris now described with respect to FIG. 6.

[0047]FIG. 6 illustrates a mass flow sensor according to anotherembodiment of the present invention in which large output transistors60, 61 are provided at the output of each of the first and secondamplifiers U₁ and U₂, respectively. The circuit of FIG. 6 also includesan amplifier or range circuit 40 that may be used to provide greaterrange for the voltage signals V_(U) and V_(D). In the embodimentdepicted in FIG. 6, the voltage signals V_(U) and V_(D) are provided toan A-D converter 50.

[0048] In contrast to the embodiment of FIG. 5, the sensor of FIG. 6includes only two switches, switch A and switch B, that are used toswitchably connect the common reference leg 30 (formed by resistors R₁and R₂) to one or the other of the upstream and downstream circuits 10,20. In the position shown in FIG. 6, switches A and B are both connectedto the upstream circuit 10, although at a different time they may beswitched to be connected to the downstream circuit 20.

[0049] In further contrast to the sensor circuit of FIG. 5 whichincluded a fixed voltage divider forming the common reference leg 30,the circuit of FIG. 6 includes a programmable voltage divider.Specifically, as depicted in FIG. 6, a temperature signal is provided toa third switch C that connects one or more resistors between the outputof the voltage divider and the reference terminal. When the switch C isin a closed state (that is, connected to the midpoint between theresistors labeled R_(2′)and R_(2″),), the voltage provided by the outputof the voltage divider is proportional to R_(2′)/(R₁+R_(2′)), whereaswhen the switch C is in an open position, or is connected to thecapacitor C_(D1), the output of the voltage divider is proportional to(R_(2′)+R_(2″))/(R₁+R_(2′)+R_(2″)). By providing a pulse-width modulatedsignal to the switch C, output voltages from the voltage divider thatare between these two values may be provided, with the division ratiobeing adjusted by the frequency and duration of the pulse-widthmodulated signal. As with the embodiment described above with respect toFIG. 5, although the ratio of R_(UR) to R_(DR) should be stable and theresistance of R_(UR) and R_(DR) preferably have the same value, it isnot required that they be identically matched.

[0050]FIG. 7 illustrates a schematic diagram of one exemplaryimplementation of a mass flow sensor according to an embodiment of thepresent invention. In FIG. 7, those portions of the circuit performingsimilar functions as described above with respect to FIGS. 5 and 6 areindicated by the same reference designators. For example, in FIG. 7, thefirst amplifier U₁ may be formed by the combination of the amplifierU53-A, capacitor C71, resistor R159, transistor Q1, and resistor R153.The downstream amplifier U₂ is formed similarly from the combination ofamplifier U53-B, capacitor C105, resistor R160, transistor Q2, andresistors R163 and R154.

[0051] In FIG. 7, the resistor R_(UR) is formed by a parallelcombination of a number of like valued resistors to achieve the desiredprecision in resistance values, as is the corresponding resistor R_(DR).It should be appreciated that other ways of providing these resistorsmay be provided, as embodiments of the present invention are not limitedto the particular implementation shown.

[0052] The common reference leg in FIG. 7 is again formed by the seriescombination of R₁ and R₂. However, in the schematic of FIG. 7, theoutput of the voltage divider may be set to provide a wide range ofvalues. Specifically, based upon a course resistance adjustment signalPWM_ICOARSE that is provided to transistor Q3, and a fine resistanceadjustment signal PWM_IFINE that is provided to transistor Q4, a rangeof resistance values may be provided for the resistor R₂. Thus, byappropriately modulating the course and fine adjustment signals, interms of frequency and duration, provided to transistors Q₃ and Q₄, arange of resistance values may be provided.

[0053] Switches 1 _(A) and 2 _(A) are used to connect the input of thevoltage divider formed by the series combination of resistors R₁ and R₂to one of the upstream and downstream circuits, while the switches 1_(B) and 2 _(B) are used to connect the output of the voltage divider tothe non inverting (+) input of one of the first and second amplifiers U₁and U₂. During the time period during which the switch 1 _(B) is closed,the capacitor C_(U1) is charged up to the value of V_(U), and during theperiod in which switch 2 _(B) is closed, the capacitor C_(D1) is chargedup to the value of V_(D). By providing a sampling signal that is delayedin time relative to the signals that selectively connect the input ofthe voltage divider to the output of one of the first and secondamplifiers, the voltage divider is permitted time to stabilize prior tothe sampling of the upstream and downstream voltage levels V_(U) andV_(D).

[0054]FIG. 8 illustrates a schematic diagram of another exemplaryimplementation of a mass flow sensor according to an embodiment of thepresent invention. In FIG. 8, those portions of the circuit performingsimilar functions as described above with respect to FIGS. 5, 6, and 7are indicated by the same reference designators. For example, in FIG. 8,the first amplifier U₁ may be formed by the combination of the amplifierU53-A, capacitor C71, resistor R159 and capacitor C146, transistor Q1,and resistor R153. The downstream amplifier U₂ is formed similarly fromthe combination of amplifier U53-B, capacitor C105, resistor R160 andcapacitor C147, transistor Q2, and resistor R154. As in the embodimentof FIG. 7, transistors Q1 and Q2 are used to provide sufficient currentto each of the upstream and downstream coils R_(U) and R_(D). In FIG. 8,each of the resistors R_(UR) and R_(DR) is again formed by a parallelcombination of a number of like valued resistors to achieve the desiredprecision in resistance values, in a manner similar to FIG. 7. It shouldbe appreciated that other ways of providing these resistors may beprovided, as embodiments of the present invention are not limited to theparticular implementation shown.

[0055] In a manner similar to the embodiment of FIG. 7, the commonreference leg 30 in FIG. 8 includes a programmable voltage divider thatmay be used to provide a wide range of resistive values and thus,division ratios. However, in contrast to the embodiment of FIG. 7, inwhich the output of the programmable voltage divider varies inaccordance with Pulse Width Modulated (PWM) control signals PWM_ICOARSEand PWM_IFINE that are provided to separate transistors Q3 and Q4, asingle multiplying Digital to Analog (D/A) converter circuit is usedinstead. In FIG. 8, the D/A converter circuit includes U50-B, U4, U13-A,and C109. In the illustrated embodiment, U4 is a sixteen bit multiplyingD/A converter that converts a voltage level to a current. The current isconverted to a variable output voltage through the use of an amplifierU13-A that is coupled to the output of the D/A converter U4. The D/Aconverter circuit feeds a single reference divider that includes R27,R139, and R166, and buffer amplifier U50-A. Each of the sample and holdcircuits (U32-A, R155, and C111(C_(U1)); U32-B, R156, and C112(C_(D1)))is switchably connected to the output of the programmable voltagedivider. With reference to FIGS. 5 and 6, the resistor R₁ may correspondto R166, and the resistor R₂ may correspond to the combination of R139,R27, and the D/A converter circuit.

[0056] It should be appreciated that the embodiment of FIG. 8 sharesmany of the same advantages as the embodiment of FIG. 7, in that eachembodiment includes a common reference leg 30 that is shared completelybetween the upstream and downstream circuits. Given equal seriesresistors upstream (R168-R171) and downstream (R172-R175), bothembodiments provide exceptionally good matching between upstream anddownstream coil resistances. However, the embodiment of FIG. 8 issignificantly faster than the embodiment of FIG. 7. For example, in oneimplementation, the embodiment of FIG. 7 exhibited a compensated risetime of approximately 110 ms (measured from 2% to 98% on the rising edgeof a fluid pulse), but it was noted that the 2% point was delayed byapproximately 30 ms from the rising edge of the fluid pulse. This delayis believed to be due to the time constant of the RC filter incorporatedinto the sample and hold circuits of FIG. 7 (i.e., U32-A, R155, and C111(C_(U1)); U32-B, R156, and C112 (C_(D1))) having a time constant ofapproximately 50 us. In contrast, an implementation of the embodiment ofFIG. 8 exhibited a compensated rise time of approximately 100 ms(measured from 2% to 98% on the rising edge of a fluid pulse), but withsubstantially less delay to the 2% point from the rising edge of thefluid pulse. This reduction in the delay is believed to be attributableto the reduction in the time constant of the RC filter incorporated intothe sample and hold circuits (i.e., U32-A, R155, and C111 (C_(U1));U32-B, R156, and C112 (C_(D1))) in the embodiment of FIG. 8 having atime constant of approximately 0.33 us. Although the embodiment of FIG.8 is significantly faster than the embodiment of FIG. 7, the embodimentof FIG. 7 provides a lower cost alternative to the embodiment of FIG. 8.Moreover, it should be appreciated that response time of the embodimentof FIG. 7 is still approximately twice that of conventional mass flowsensors.

[0057]FIGS. 9A and 9B illustrate a mass flow sensor according to anotheraspect of the present invention in which only a portion of the referenceleg is common to both the upstream and downstream circuits 10, 20. As ineach of FIGS. 5, 6, 7, and 8, the reference designator R_(U) representsthe upstream coil or resistor, and reference designator R_(D) representsthe downstream coil or resistor. As previously described, coils R_(U)and R_(D) may, for example, be disposed at spaced apart positions abouta sensor conduit (not shown) through which a fluid flows, with each coilR_(U) and R_(D) having a large and substantially identical thermalcoefficient of resistance, such that the resistance of each coils R_(U),R_(D) varies with temperature. It should again be appreciated thatalthough the upstream and downstream coils are referred to as “coils,”the present invention is not so limited. Moreover, although embodimentsof the present invention are described in terms of a mass flow sensor,the present invention is not so limited, as aspects of the presentinvention may be used in other applications in which variations in theresistance of a leg of a resistive bridge circuit is indicative of achange in a property that varies with resistance.

[0058]FIG. 9A illustrates a simplified schematic diagram of a mass flowsensor in which only a portion of a reference leg is shared betweenseparate upstream and downstream circuits. The sensor includes anupstream resistive bridge circuit 10 and a downstream resistive bridgecircuit 20 that are used to set the resistance and, thus, thetemperature of the upstream coil R_(U) and the downstream coil R_(D),respectively.

[0059] The upstream circuit 10 includes a first amplifier U₁, a seriesconnection of a first resistor R_(UR) and a first variable resistorR_(U) (the upstream coil) electrically connected between an output ofthe first amplifier U₁ and a reference terminal. Electrically connectedbetween the output of the first amplifier U₁ and the inverting (−) inputof the first amplifier U₁ is a series connection of a capacitor C_(U2)and a resistor R_(U2). Electrically connected between the mid-point ofthe first resistor R_(UR) and the first variable resistor R_(U) and theinverting input of the first amplifier U₁ is another resistor R_(U1). Arelatively large valued capacitor C_(U1) is electrically connectedbetween the non-inverting (+) input of the first amplifier U₁ and thereference terminal. A resistor R_(2U) is connected in parallel with thecapacitor C_(U1) between the non-inverting (+) input of the firstamplifier U₁ and the reference terminal. The resistor R_(2U) forms aportion of the reference leg of the upstream circuit 10.

[0060] The downstream circuit 20 is similar to the upstream circuit 10.The downstream circuit 20 includes a second amplifier U₂, and a seriesconnection of a second resistor R_(DR) in series with a second variableresistor R_(D) (the downstream coil) that is connected between theoutput of the second amplifier U₂ and the reference terminal. Theinverting (−) input of the second amplifier U₂ is electrically connectedto a mid-point of the series connection of the second resistor R_(DR)and the second variable resistor R_(D) through a resistor R_(D1), and acapacitor C_(D2) in series with a resistor R_(D2) is electricallyconnected between the output of the second amplifier U₂ and theinverting input (−) of the second amplifier U₂. The non-inverting (+)input of the second amplifier U₂ is electrically connected a largevalued capacitor C_(D1) that is connected to the reference terminal, anda resistor R_(2D) is connected in parallel with the capacitor C_(D1)between the non-inverting (+) input of the second amplifier U₂ and thereference terminal. The resistor R_(2D) forms a portion of the referenceleg of the downstream circuit 20.

[0061] As shown in FIG. 9A, the circuit further includes a commonresistor R₁ that is switchably connected to one of the upstream anddownstream circuits 10, 20 via a respective switch 1 _(A) and 2 _(A).The common resistor R₁ is electrically connected in series with one ofresistor R_(2U) or R_(2D) via switches 1 _(B) and 2 _(B) to form thereference leg of each of the upstream and downstream circuits 10, 20.The reference leg that is formed by the series connection of R₁ with oneof R_(2U) and R_(2D) sets the value of resistance to which the upstreamcoil R_(U) and the downstream coil R_(D) are set and acts as a voltagedivider. As shown, switches 1 _(A) and 2 _(A) are connected between aninput of the voltage divider formed by the series connection R₁ and oneof R_(2U) and R_(2D) and the output of the first and second amplifiersU₁, U₂, respectively. Switches 1 _(B) and 2 _(B) are each respectivelyconnected to the output of the voltage divider and the non-invertinginput of the first and second amplifiers U₁ and U₂, respectively.Switches 1 _(A) and 1 _(B) and switches 2 _(A) and 2 _(B) work intandem, such that switches 1 _(A) and 1 _(B) and switches 2 _(A) and 2_(B) are both open or closed at the same time.

[0062] During operation, switches 1 _(A), 1 _(B), and switches 2 _(A), 2_(B) are alternately opened and closed to connect the common resistor R₁to one of the upstream and downstream circuits 10, 20. During the timeinterval in which the common resistor R₁ is not connected to theupstream circuit 10 (i.e., when switches 1 _(A), 1 _(B) are open) thecapacitor C_(U1) maintains the voltage level at the non-inverting inputof the first amplifier U₁. Similarly, during the time interval in whichthe common resistor R₁ is not connected to the downstream circuit 20(i.e., when switches 2 _(A) and 2 _(B) are open), the capacitor C_(D1)maintains the voltage level at the non-inverting input terminal of thesecond amplifier U₂.

[0063] In operation, the sensor behaves as two constant temperaturedriver circuits sharing a portion of a reference leg. Switches 1 and 2are switched rapidly to connect the reference leg (R₁ and one of R_(2U)or R_(2D)) to the upstream and downstream circuits alternately. C_(U1)and C_(D1) hold the sampled reference feedback when the correspondingswitches are open. The first amplifier U₁ servos such thatR_(U)/R_(UR)=R_(2U)/R₁. The second amplifier U₂ servos so thatR_(D)/R_(DR)=R_(2D)/R₁. Other amplifiers (not shown in FIG. 9A) pick offthe upstream and downstream voltage levels V_(U) and V_(D) between theseries connection of the first resistor R_(UR) and the first variableresistor R_(U) and the series connection of the second resistor R_(DR)and the second variable resistor R_(D). The voltage levels V_(U) andV_(D) can then be used to provide a signal that is indicative of theflow rate of fluid through the conduit in which, or about which, theupstream and downstream coils R_(U), R_(D) are disposed. For example, incertain embodiments, the ratio of (V_(U)−V_(D))/V_(D) or(V_(U)−V_(D))/V_(U) may be used to provide the signal that is indicativeof the flow rate of fluid. In other embodiments, described more fullybelow, the voltage levels V_(U) and V_(D) may be combined to provide aratio of (V_(U)−V_(D))/(V_(U)+V_(D)) which is also indicative of theflow rate of the fluid, but provides a signal having a range that issymmetric independent of the direction of the flow of fluid (e.g., fromthe upstream coil to the downstream coil, or from the downstream coil tothe upstream coil). The remaining components illustrated in FIG. 9A,namely R_(U1), R_(U2), C_(U2), R_(D1), R_(D2), and C_(D2), are used tostabilize the first and second amplifiers U₁ and U₂.

[0064] It should be appreciated that the circuit of FIG. 9A isfunctional in nature and may be modified in a variety of ways. Forexample, high power amplifiers may be used to provide an appropriateamount of current to the upstream and downstream coils. Alternatively,the output of the first and second amplifiers may be electricallyconnected to a large output transistor to provide an appropriate amountof current. Moreover, this embodiment is not limited to the use of fourswitches 1 _(A), 1 _(B), and switches 2 _(A), 2 _(B), as fewer switchesmay be used. It should also be appreciated that in variousimplementations, the reference leg formed by R₁ and one of R_(2U) andR_(2D) may be replaced with a programmable voltage divider. Thus, withappropriate control of the programmable voltage divider, a programmabletemperature rise sensor driver may be provided. An embodiment of a flowsensor that includes a programmable voltage divider is now describedwith respect to FIG. 9B.

[0065]FIG. 9B illustrates a schematic diagram of an exemplaryimplementation of a mass flow sensor in which separate upstream anddownstream circuits share only a portion of a reference leg according toan embodiment of the present invention. In FIG. 9B, those portions ofthe circuit performing similar functions as described above with respectto FIG. 9A are indicated by the same reference designators. For example,in FIG. 9B, the first amplifier U₁ may be formed by the combination ofthe amplifier U53-A, capacitor C71, resistor R159 and capacitor C146,transistor Q1, and resistor R153. The downstream amplifier U₂ is formedsimilarly from the combination of amplifier U50-A, capacitor C105,resistor R160 and capacitor C147, transistor Q2, and resistor R154. Asin the embodiments of FIGS. 7 and 8, transistors Q1 and Q2 are used toprovide sufficient current to each of the upstream and downstream coilsR_(U) and R_(D).

[0066] In FIG. 9B, each of the resistors R_(UR) and R_(DR) is againformed by a parallel combination of a number of like valued resistors toachieve the desired precision in resistance values, in a manner similarto FIGS. 7 and 8. However, it should be appreciated that other ways ofproviding these resistors may be provided, as embodiments of the presentinvention are not limited to the particular implementation shown.

[0067] In the embodiment of FIG. 9B, the shared resistor R₁ that iscommon to each of the upstream and downstream circuits is a variableresistor that can be used to form a programmable voltage divider incombination with R_(2U) and R_(2D) of the upstream and downstreamcircuits to permit a range of resistive values and thus, divisionratios. For example, reference leg division ratios that may be providedby the embodiment of FIG. 9B may vary from approximately 0.770 to 0.834.In the illustrated embodiment, the common shared resistor R₁ includes amultiplying Digital to Analog (D/A) converter circuit that includes U4,U13-A, U13-B, C109, and resistors R110 and R113. U4 is a sixteen bitmultiplying D/A converter that converts a voltage level to a current.The current provided by the D/A converter is converted to a variableoutput voltage through the use of amplifiers U13-A and U13-B andresistors R110 and R113. Common shared resistor R₁ is connected to aselector switch, illustrated as a pair of switches (U6-A, U6B), thatalternately connect the shared resistor R₁ to upstream and downstreamdrive signals (e.g., the drive voltage signal at the emitters of Q1 andQ2, hereinafter referred to as “the drive voltage”). The capacitor C107is used to eliminate a narrow voltage spike when switching betweenupstream and downstream sources of the drive signals.

[0068] In contrast to each of the previously described embodiments ofFIGS. 5-8, each of the upstream and downstream circuits of FIG. 9B alsoincludes its own portion of the reference leg. For the upstream circuit,this portion of the reference leg (denoted R_(2U) in FIG. 9A) includesresistors R166, R139, and R27, and for the downstream circuit, thisportion of the reference leg (denoted R_(2D) in FIG. 9A) includes R127,R136, and R114. Each of these reference leg portions is switchablyconnected to the common shared resistor R₁ by a respectivesample-and-hold circuit (U32-A, R155, C111 (C_(U1)), and U53B upstream;and U32-B, R156, C112 (C_(D1)), and U50-B downstream). As in theprevious embodiments, during the time interval in which the referenceleg is not connected to the upstream circuit (i.e., when switches 1_(A), 1 _(B) are open), the hold capacitor C_(U1) (C111) maintains thevoltage level at the non-inverting input of the first amplifier U₁.Similarly, during the time interval in which the reference leg is notconnected to the downstream circuit (i.e., when switches 2 _(A) and 2_(B) are open), the hold capacitor C_(D1) maintains the voltage level atthe non-inverting input terminal of the second amplifier U₂.

[0069] Recalling that each of the upstream and downstream sensor coilsR_(U) and R_(D) are one leg of a bridge circuit, the sensor circuit ofFIG. 9B operates in the following manner. The associated upstream anddownstream driver amplifier (U₁, U₂) controls the resistance of itsassociated sensor coil R_(U) and R_(D) (and thus its temperature) bychanging the drive voltage to the bridge. Increasing the drive voltageincreases the current through the sensor coil, causing the sensor coilto warm up. Each bridge includes a reference leg (the series combinationof R₁ and R_(2U), or the series combination of R₁ and R_(2D)) thatprovides some temperature-dependent division ratio, and a sensor legconsisting of the sensor coil plus lead Rc and a series resistor Rs(i.e., R_(UR) or R_(DR)). For the upstream circuit 10, the sensor legconsists of the upstream coil R_(U) plus its lead and the resistorR_(UR); for the downstream circuit, the sensor leg consists of thedownstream coil R_(D) Plus its lead and the resistor R_(DR). The sensorleg has a division ratio of Rc/(Rc+Rs) and the drive amplifiercontinually servos, attempting to match that division ratio to thedivision ratio of the reference leg formed by the series combination ofR₁ and R_(2U), or the series combination of R₁ and R_(2D). The D/Aconverter circuit (U4, U13-A, U13-B, C109, R110, and R113) feeds aprogrammable fraction of each side's drive voltage to that side'ssample-and-hold circuit (U32-A, R155, C111, and U53B upstream; U32-B,R156, C112, and U50-B downstream).

[0070] Assuming an ideal circuit, with resistor values as shown on FIG.9B and ideal amplifiers, etc., when the D/A converter is set to 0, thevoltage at the output of U13-B is 0. Accordingly, both sample-and-holdcircuits produce 0 V at their outputs (U53-B upstream, U50-Bdownstream). The non-inverting input (+) of each drive amplifier (U₁,U₂) sees 0.77032 times the corresponding drive voltage, and servos toset the sensor coil+lead resistance to 3.3539 times the correspondingRs. When the D/A converter is set to 1, the voltage at the output ofeach sample-and-hold circuit is equal to the corresponding drivevoltage. The non-inverting (+) input of each drive amplifier (U₁, U₂)sees 0.83398 times the corresponding drive voltage, and servos to setthe sensor coil+lead resistance to 5.0235 times the corresponding Rs.Intermediate D/A settings provide intermediate sensor coil+leadresistances linearly proportional to the D/A setting.

[0071] It should be appreciated that sensor drive circuit of FIG. 9Bprovides a number of advantages over conventional sensor drive circuits.For example, when the drive voltage changes, the voltage at thenon-inverting (+) input of the drive amplifier (U₁, U₂) sees themajority of the ultimate change immediately. For a D/A setting of 0, itsees the entire change immediately, and for a D/A setting of 1.0, itsees ˜92% (0.77032/0.83398) of the total change immediately. Theremaining 8% shows up over the next few hundred microseconds as thechange propagates through the sample and hold circuits. Further, becausethe non-inverting input of the drive amplifier (U₁, U₂) sees changes inthe drive voltage almost immediately, this permits the time constant tobe raised in each of the sample and hold circuits without negativelyimpacting response time. In current implementations, the time constantsof the sample and hold circuits have been increased to about 40 us, butit is believed that these values may be increased further. It should beappreciated that by increasing the time constant in the sample and holdcircuits, high-frequency noise in the reference leg is prevented fromstrongly affecting the sample-and-hold circuits. This, in turn,drastically reduces the noise level in the resulting flow signal formedfrom the signals V_(U) and V_(D). Additionally, any residual noise lefton the hold capacitor (i.e., C_(U1) and C_(D1)) when the sample switch(i.e., switch 1 _(B) and 2 _(B)) opens no longer strongly affects thevoltage at the non-inverting (+) input to the drive amplifier.Specifically, with the component values shown in FIG. 9B, a 1 mV erroron the hold capacitor turns into less than 64 uV error at the driveamplifier's non-inverting (+) input. This also helps substantiallyreduce the noise level in the resulting flow signal.

[0072] In initial testing of an exemplary implementation of theembodiment of FIG. 9B, compensated rise times of approximately 60 ms(measured from 2% to 98% on the rising edge of a fluid pulse) wererepeatedly obtained, with no excess driver delay. Moreover, whenimplementations of the embodiment depicted in FIG. 9B were incorporatedinto a mass flow controller, settling times of 100 to 130 ms within 2%of the desired final value were obtained. It should be appreciated thatthe above results correspond to a settling time that approximately oneeighth that of conventional mass flow controllers.

[0073] Referring back to the embodiment depicted in FIG. 9B, thisembodiment includes several components that have not been previouslydescribed in detail. For example, resistors R159 and R160 are providedto limit transistor base current in output transistor Q1 and Q2 duringstart-up and over-flow conditions, when the output of the driveramplifiers (U₁, U₂) exceeds the sensor supply voltage (at the collectorsof Q1 and Q2) by a substantial amount. Back to back diodes CR14 and CR15prevent damage to the base-emitter junctions of Q1 and Q2 if the outputsof the drive amplifier U153-A and U50-A (U₁, U₂) go to the negativesupply rail for any reason. Capacitors C146 and C147 are not used atpresent but may be used to implement future changes. Resistors R153 andR154 provide a small amount of sensor current at start-up, to ensurethat amplifier offsets don't lead to the drive amplifiers trying todrive to a negative voltage. U33 and U34 convert a single pulse train(PWM_FLOW) into properly-phased signals USelect, USample, and DSampleused to drive switches 1 _(A) and 2 _(A) and 1 _(B) and 2 _(B) (U6 andU32).

[0074] Resistors R140 and R141, and capacitors C124 and C125 are used toprovide a fast response under certain circumstances. In particular, inearly prototypes wherein resistors R27 and R114 were connected directlyto ground, the circuit was difficult to stabilize, requiring very low ACgain in the driver compensation circuits (C71, C106, R22 and R20upstream; and C105, C108, R57 and R21 downstream). Using high AC gain(R27/R20, R114/R21) produced oscillation with both driver amplifiersslamming rail-to-rail 180 degrees out of phase. Adding resistors R140and R141, and capacitors C124 and C125 prevents the entire change indrive voltage from showing up at the drive amplifier non-inverting (+)input immediately. With the values shown, and the D/A set to 0,approximately 98.5% of the ultimate change occurs immediately, with theremaining 1.5% arriving with a time constant of ˜500 us. This veryslight lag in the non-inverting (+) input signal allows much higher ACgain in the drive amplifier, without making the driver unstable, thusallowing much faster response to flow changes.

[0075] It should of course be appreciated that the specific componentvalues shown in FIG. 9B are specific to the desired operationalcharacteristics of the mass flow sensor, and the range of conditionsunder which it is intended to operate. Thus, for sensors designed fordifferent rates of flow and/or different operating conditions, it shouldbe appreciated that the component values shown in FIG. 9B may beadjusted accordingly. It should also be appreciated that other changesto the implementation shown in FIG. 9B may also be readily envisioned.For example, for lower cost implementations, the multiplying D/Aconverter circuit of FIG. 9B (including U4, U13A, U13-B, C109, andresistors R110 and R113) may be replaced with a D/A converter circuitsimilar to that of FIG. 7, in which pulse width modulated controlsignals are used to vary the output voltage provided by the voltagedivider.

[0076] In each of the embodiments of FIGS. 5-9B described above,although the ratio of R_(UR) to R_(DR) should be stable and theresistance of R_(UR) and R_(DR) preferably have the same value, it isnot required that they be identically matched. Accordingly, each of theembodiments described with respect to FIGS. 5-9B above dispense with theneed to closely match component values and characteristics as requiredby the circuits of FIGS. 2-4.

[0077] According to another aspect of the present invention, anamplifier circuit is provided for amplifying first and second signalsprovided by a sensor. The amplifier circuit provides an output signalhaving a range that is symmetric independent of the orientation of thesensor. Advantageously, this aspect of the present invention may be usedwith each of the embodiments described with respect to FIGS. 5-9 above.

[0078] Referring back to FIGS. 5-9, it should be appreciated thatalthough the upstream and downstream coils R_(U) and R_(D) are similarin construction and electrical and thermal properties, operation of thesensor circuit may vary dependent upon the direction of flow. That is,depending on the specific combination of upstream and downstream voltagelevels (V_(U), V_(d)) that are used to detect flow, the sensor drivermay perform very differently when the direction of flow is reversed andthe “upstream” coil is used as the “downstream” coil and vice versa. Forexample, where the flow signal indicative of the rate of flow of fluidthrough the mass flow sensor is calculated based upon the equation:

Flow =K*(Vu−Vd)/Vd;  (1)

[0079] the range of the flow signal may vary dependent upon which of thecoils R_(U) and R_(D) is used as the upstream coil, and which is used asthe downstream coil. Although this flow signal is independent (to afirst approximation) of ambient temperature (ignoringtemperature-dependent fluid and sensor material thermal properties), thepower supplied to the upstream and downstream coils varies as a functionof flow. As a result, when the mass flow rate of the fluid is calculatedbased upon equation 1, the resulting flow signal is highly asymmetric,having a linear range that is substantially larger in one direction(e.g., when the flow is from the upstream coil to the downstream coil)than the other.

[0080] However, according to a further aspect of the present invention,rather than calculating the mass flow rate of the fluid based uponequation 1, the flow rate may be instead be calculated as:

Flow K*(Vu−Vd)/(Vu+Vd).  (2)

[0081] The above definition of the flow rate of fluid is also (ignoringtemperature-dependent fluid and sensor material thermal properties)independent of temperature. However, because (Vu−Vd) and (Vu+Vd) areboth symmetric functions of flow, the flow signal is also symmetric.This symmetry permits the sensor driver circuit to perform equally wellwith flow in either direction. Thus, when it is desired to use thesensor in a reversed orientation wherein the “upstream” coil (R_(U)) isoriented downstream of the “downstream” coil (R_(D)), it is notnecessary to physically reverse the sensor, or to change the electronicsto compensate for the reversed direction of flow. Instead, the outputsignal provided by the sensor circuit may simply be inverted inside thedigital signal processor (not shown) that processes the output signal.Another definition of flow rate that is a symmetric function of flow isthe difference between the upstream and downstream voltages, that isVu−Vd. In certain embodiments, this latter definition of flow rate maybe preferred to that set forth in equation two above, as it isindependent of which coil is oriented upstream of the other, but is lesssusceptible to noise and provides an increased sensitivity at lowerambient temperatures.

[0082] Referring to FIG. 10, an amplifier circuit that provides anoutput signal having a range that is symmetric independent of theorientation of the sensor is now described. The amplifier circuitincludes a pair of amplifiers U30 and U₁ 7-B that each receives theoutput signals V_(U), V_(D) from the sensor circuit. The amplifiercircuit provides a differential numerator signal (the difference betweensignals labeled “DELTAV+” and “DELTAV−”) that is equal to K1*(Vu−Vd).This differential numerator signal may be applied to the differentialsignal inputs of an A/D converter (ADC) that converts this signal to adigital value for subsequent processing by a Digital Signal Processor(DSP) of a mass flow controller (not shown). The amplifier circuit alsoprovides a single-ended denominator signal (labeled “VD+VU”) that isequal to K2*(Vu+Vd) and which may be applied to a single-ended referenceinput of the A/D converter (ADC). The output of the A/D converter thusprovides a digital output signal that is equal to K*(Vu−Vd)/(Vu+Vd) thatmay be further processed by the DSP of the mass flow controller.

[0083] The amplifier circuit of FIG. 10 may be used with any of theembodiments of the sensor circuits described above with respect to FIGS.5-9, as each of these circuits is capable of providing sensor outputsignals V_(U) and V_(D) that represent the voltage across the upstreamand downstream coils. For example, in FIG. 7, the lower right handportion of the circuit denoted “Flow Sensor Amp” may be replaced withthe amplifier circuit depicted in FIG. 10 to provide a flow signalhaving a range that is symmetric independent of the orientation of thesensor. Moreover, it should be appreciated that that other amplifiercircuits may be used that differ from the implementation shown in FIG.10, as the present invention is not so limited. Indeed, as long as theoutput of the amplifier circuit provides signals that are indicative ofthe difference in voltage (or current) provided to the upstream anddownstream coils, and some combination of these signals that isindependent of the direction of the flow of fluid through the sensor isused to detect their difference, the range of the flow signal will besymmetric independent of the orientation of the sensor. Where symmetryin the flow signal independent of sensor orientation is not required,various combinations of the sensor output signals V_(U) and V_(D) may beused, including (V_(U)−V_(D))/V_(U) and (V_(U)−V_(D))/V_(D).

[0084] According to a further aspect of the present invention, avariable output power supply is provided that is capable of regulatingthe amount of power used by a sensor circuit. According to oneembodiment in which the variable output power supply is used with a massflow sensor circuit, the variable output power supply is capable ofproviding an output that varies in response to a detected flow rate,such that more power is provided to the mass flow sensor circuit at highflow rates than at low flow rates. It should be appreciated inconventional mass flow sensor circuits, as much as 50% of the powerprovided to the mass flow sensor is wasted at low flow rates. Accordingto another embodiment, the variable output power supply is capable ofpreventing the amount of power supplied by the variable output powersupply from increasing excessively at very high flow rates, and forpreventing phase reversal and potential latch-up in control systemsassociated with the mass flow sensor circuit. Advantageously, both ofthese aspects may be incorporated into a single variable output powersupply. Such an embodiment is now described with respect to FIG. 11.

[0085] The variable output power supply 1100 includes a partiallyisolated switching power supply 1110 that provides a variable outputvoltage, controlled by a control circuit 1120 (denoted “7 V Control”).The positive output of the partially isolated switching power supply1110 (the signal labeled “+7V”) provides power to both sensor drivecircuit transistors (i.e., transistors Q1 and Q2 in FIGS. 7, 8, and 9B).Return current from the sensor flows back to the partially isolatedswitching power supply 1110 through the “CABLE_SENSE” line (pin 3 on thesensor in FIGS. 8 and 9B). A separate lead to the sensor common point(pin 9 of the sensor in FIGS. 8 and 9B) provides a ground reference tothe variable output power supply 1100.

[0086] The variable output power supply 1100 provides a voltage on the“+7V” line that is 1V greater than the greatest sensor drive voltage(defined previously above as the voltage at the emitter of either drivetransistor Q1, Q2, and not the upstream or downstream sensor coilvoltage V_(u) or V_(d)). This provides a minimum 1V difference betweenthe collector and the emitter (Vce) on both drive transistors (Q1 and Q2in FIGS. 7,8, and 9B) that provides sufficient drive current to preventsaturation of either transistor, but without wasting a lot of power inthe drive transistors.

[0087] Ignoring for a moment the operation of U8-A and the right half ofdual diode CR6, dual diode CR5 and resistor R60 produce a voltage (atpin 3 of CR5) of roughly one diode drop below the highest “drivevoltage.” The left half of dual diode CR6 and resistor R59 translatethis back up one diode drop, to produce a voltage at pin 3 of CR6 thatis roughly equal to the highest “drive voltage.” Amplifier U8-B and itsassociated passive components then translate this voltage to provide anoutput voltage (on the “+7V” line ) that is one volt greater than thevoltage at pin 3 of CR6.

[0088] It should be appreciated that the variable output power supplythus provides an output voltage that is just slightly greater than thatrequired for proper operation of the sensor circuit, raising andlowering the supply voltage provided to the sensor circuit as needed inresponse to the actual power consumption of the sensor driver circuit.It should also be appreciated that this aspect of the present inventionis equally applicable to each of the sensor circuits of FIGS. 7, 8, and9B, and may be used with other sensor circuits whenever powerconsumption is a consideration. Indeed, where power consumption is ofprimary concern, and cost is not an issue, rather than providing asingle output voltage to both the upstream and downstream sensorcircuits, separate power supply circuits may be provided. For example,the upstream sensor driver circuit may have its own variable outputpower supply, and the downstream sensor driver circuit may have its ownvariable output power supply, each similar to that described above withrespect to FIG. 11.

[0089] According to another aspect of the present invention, thevariable output power supply can also be capable of preventing theamount of power supplied by the variable output power supply fromincreasing excessively at very high flow rates, and for preventing phasereversal and potential latch-up in control systems associated with themass flow sensor circuit. As known to those skilled in the art, at highflow rates, the fluid flow through the sensor may be too fast to beheated properly, thereby sucking power out of both upstream anddownstream sensor coils. This can have two negative effects. First, theoutput of the sensor circuit begins to decrease with increasing flowrates, leading to a phase reversal in most control systems andconsequent latch-up if uncorrected. Second the sensor circuit powerconsumption increases dramatically at high flow rates, in some casesmore than doubling compared to the zero-flow power consumption. Becausethe output of the sensor circuit decreases at very high flow rates, andbecause this can happen almost instantaneously, it is generally notpossible to detect high-flow conditions by monitoring only the output ofthe sensor circuit.

[0090] According to an aspect of the present invention, a method ofdetecting high flow conditions in a mass flow sensor is provided. Themethod includes acts of calculating an expected zero flow signal at thecurrent operating temperature, calculating a threshold based upon theexpected zero flow signal, comparing an actual flow signal to thethreshold, and detecting the high flow condition when the actual flowsignal exceeds the threshold. This method may be implemented by amicroprocessor, which advantageously may be the same microprocessor asthat used in the mass flow controller that includes the mass flow sensorcircuit.

[0091] According to one embodiment, the expected zero flow signal iscalculated according to the sum of the upstream and downstream coilvoltages (V_(U), V_(d)) times a constant (K). That is:

Expected Zero Flow =K*(V _(u) +V _(d));

[0092] at zero flow and the current operating temperature. The thresholdis determined by multiplying a constant (typically 1.05 to 1.10) timesthe Expected Zero Flow signal. Based upon a comparison of the thresholdand the actual flow signal (K*(V_(u)+V_(d))), a determination is made asto whether the high flow condition exists. When it is determined thatthe high flow condition exists, the sensor supply voltage provided bythe +7V line of the variable output power supply is prevented fromincreasing excessively. In addition, when it is determined that the highflow condition exists, the indicated sensor circuit output isartificially set to a high (positive or negative, depending on flowdirection) value to prevent latch-up of the associated control system(typically some sort of an Integral (I), Proportional Integral (PI),Proportional Integral Differential (PID), Lead Lag (LL), Gain Lead Lag(GLL), etc. control system implemented by a microprocessor, for example,the microprocessor of a mass flow controller). The microprocessorprovides a digital output signal that is converted to a pulse widthmodulated signal (PWM_SUPPLY in FIG. 11) to limit the supply voltageprovided by the variable output power supply to the sensor circuit.

[0093] In the embodiment depicted in FIG. 11, a PWM (pulse-widthmodulator, not shown) is used to drive an RC filter that includesresistor R37 and capacitor C35. Whenever the actual flow signal(K*(Vu+Vd)) is below the threshold value, the output provided by themicroprocessor is set to the maximum possible value. This output isprovided to the PWM, and the output of the PWM (PWM_SUPPLY) forces theoutput of amplifier U8-A high, reverse-biasing the right half of dualdiode CR6, and allowing the partially isolated switching power supply1110 to operate normally. However, whenever the actual flow signal(K*(V_(u)+V_(d))) exceeds the threshold value, the output of themicroprocessor is reduced proportionately. As the output drops, so doesthe voltage at the output of-amplifier U8-A. At some point, the righthalf of dual diode CR6 turns on, reducing the voltage at pin 3 of CR6below the normal value, and thus reducing the output of the partiallyisolated switching power supply 1110. This prevents the actual flowsignal (K*(V_(u)+V_(d))) from increasing further, artificially coolingthe sensor and substantially reducing the sensor driver powerconsumption below what it would be otherwise.

[0094] When the flow through the sensor decreases, the sensor will warmback up (since the available power being provided to the sensor isgreater than that required for normal operation), and the actual flowsignal (K*(V_(u)+V_(d))) will eventually drop below the threshold. Themicroprocessor will then return the output to its normal high value,allowing the partially isolated switching power supply 1110 to resumenormal operation.

[0095] It should be appreciated that although this embodiment wasdescribed with reference to a pulse width modulator, other circuitelements may be used instead. For example, rather than using a PWM as aD/A converter to convert the digital output signal of the microprocessorto an analog value, other types of D/A converters may be used as well.Moreover, it should also be appreciated that although the describedembodiment limits the voltage that is provided to the sensor circuit, itcould alternative limit the supply current instead. In addition, ratherthan using the combination of (V_(u)+V_(d)) as the basis for thecomparison, one may alternatively monitor other signals, such as thedrive voltage provided to each or both sensors, or the sensor currentprovided to each or both sensors, etc. In this regard, the combinationof (V_(u)+V_(d)) is used because this signal is already available fromthe output of the flow sensor amplifier depicted in FIG. 10, but othercombinations of signals may be used instead.

[0096] Although embodiments of the present invention have been describedwith respect to a mass flow sensor that is particularly well suited forsemiconductor manufacturing processes, it should be appreciated thatembodiments of the present invention may be used in other applicationsand processes. For example, embodiments of the present invention may beused in automotive applications to measure the amount of a fluid such asgasoline, or diesel fuel, or air that is delivered to a combustionchamber. Moreover, embodiments of the present invention are not limitedto mass flow sensors, as the present invention may be used in othersensor and detection circuits. For example, embodiments of the presentinvention may be readily adapted for use in a hot-wire anemometer or anyother applications in which variations in the resistance of a leg of aresistive bridge circuit is indicative of a change in a property thatvaries with resistance.

[0097] Having described several embodiments of the invention in detail,various modifications and improvements will readily occur to thoseskilled in the art. Such modifications and improvements are intended tobe within the scope of the invention. In particular, although many ofthe embodiments described herein involve specific combinations of systemelements or method acts, it should be understood that those elements andacts may be combined in other ways. Thus, elements, acts, or featuresdiscussed only in connection with one embodiment are not intended to beexcluded from other embodiments. Accordingly, the foregoing descriptionis by way of example only, and is not intended as limiting. Theinvention is limited only as defined by the following claims and theequivalents thereto.

What is claimed is:
 1. A sensor, comprising: a first amplifier having afirst input, a second input, and an output; a first resistorelectrically connected in series with a first variable resistor betweenthe output of the first amplifier and a reference terminal, the firstresistor being electrically connected between the first input of thefirst amplifier and the output of the first amplifier, and the firstvariable resistor being electrically connected between the firstresistor and the reference terminal; a second amplifier having a firstinput, a second input, and an output; a second resistor electricallyconnected in series with a second variable resistor between the outputof the second amplifier and the reference terminal, the second resistorbeing electrically connected between the first input of the secondamplifier and the output of the second amplifier, and the secondvariable resistor being electrically connected between the secondresistor and the reference terminal; and a voltage divider having aninput that is switchably connected to one of the output of the firstamplifier and the output of the second amplifier, and an output that isswitchably connected to one of the second input of the first amplifierand the second input of the second amplifier, the output of the voltagedivider setting a resistance of the first variable resistor when theinput of the voltage divider is connected to the output of the firstamplifier and the output of the voltage divider is connected to thesecond input of the first amplifier, and setting a resistance of thesecond variable resistor when the input of the voltage divider isconnected to the output of the second amplifier and the output of thevoltage divider is connected to the second input of the secondamplifier.
 2. The sensor of claim 1, wherein the voltage dividerincludes a programmable voltage divider.
 3. The sensor of claim 2,wherein the programmable voltage divider includes a plurality ofresistors connected between an output of the voltage divider and thereference terminal, and the output voltage of the voltage divider can bevaried based upon which of the plurality of resistors are connectedbetween the output of the voltage divider and the reference terminal. 4.The sensor of claim 3, wherein the output voltage of the voltage dividercan further be varied based upon an amount of time each of the pluralityof resistors are connected between the output of the voltage divider andthe reference terminal.
 5. The sensor of claim 2, wherein theprogrammable voltage divider includes a digital to analog convertercircuit having an output that sets the output of the voltage divider. 6.The sensor of claim 5, wherein the digital to analog converter circuitincludes: a digital to analog converter having an output that provides avariable amount of current; and an amplifier circuit having an inputelectrically coupled to the output of the digital to analog converter,and an output that forms the output of the analog to digital convertercircuit and provides a variable output voltage based upon the variableamount of current.
 7. The sensor of claim 1, further comprising a firstcapacitor electrically connected between the second input of the firstamplifier and the reference terminal that maintains a voltage level atthe second input of the first amplifier when the output of the voltagedivider is connected to the second input of the second amplifier.
 8. Thesensor of claim 7, further comprising a second capacitor electricallyconnected between the second input of the second amplifier and thereference terminal that maintains a voltage level at the second input ofthe second amplifier when the output of the voltage divider is connectedto the second input of the first amplifier.
 9. The sensor of claim 1,wherein the second input of the first and second amplifiers isrespectively connected to a first switch and a second switch each havingan open state and a closed state, wherein a voltage level at the secondinput of the first and second amplifiers is sampled when the firstswitch and the second switch are in the closed state.
 10. The sensor ofclaim 9, wherein the first switch receives a first switching signal thatswitches the first switch to the closed state after the input of thevoltage divider is connected to the output of the first amplifier andthe output of the voltage divider is connected to the second input ofthe first amplifier.
 11. The sensor of claim 10, wherein the secondswitch receives a second switching signal that switches the secondswitch to the closed state after the input of the voltage divider isconnected to the output of the second amplifier and the output of thevoltage divider is connected to the second input of the secondamplifier.
 12. The sensor of claim 1, wherein the sensor is a mass flowsensor.
 13. The sensor of claim 12, wherein the mass flow sensor isincluded in a mass flow controller.
 14. The sensor of claim 1, whereinthe voltage divider includes: a third resistor electrically connectedbetween the input and the output of the voltage divider, the thirdresistor being switchably connected to one of the output of the firstamplifier and the output of the second amplifier; a fourth resistorelectrically connected between the second input of the first amplifierand the reference terminal; and a fifth resistor electrically connectedbetween the second input of the second amplifier and the referenceterminal.
 15. The sensor of claim 14, further comprising a firstcapacitor electrically connected between the second input of the firstamplifier and the reference terminal that maintains a voltage level atthe second input of the first amplifier when the output of the voltagedivider is connected to the second input of the second amplifier. 16.The sensor of claim 15, further comprising a second capacitorelectrically connected between the second input of the second amplifierand the reference terminal that maintains a voltage level at the secondinput of the second amplifier when the output of the voltage divider isconnected to the second input of the first amplifier.
 17. A sensorcomprising: a first circuit including a first resistor having a firstresistance that varies in response to a change in a physical property; asecond circuit including a second resistor having a second resistancethat varies in response to the change in the physical property; avoltage divider; and at least one switch having a first state and asecond state, the first state of the at least one switch electricallyconnecting the voltage divider to the first circuit to set theresistance of the first resistor, and the second state of the at leastone switch electrically connecting the voltage divider to the secondcircuit to set the resistance of the second resistor.
 18. The sensor ofclaim 17, wherein the voltage divider has an input and an output, andwherein the at least one switch includes at least one first switch andat least one second switch each having the first state and the secondstate, the at least one first switch electrically connecting the inputof the voltage divider to the first circuit when the at least one firstswitch has the first state and electrically connecting the input of thevoltage divider to the second circuit when the at least one first switchhas the second state, and the at least one second switch electricallyconnecting the output of the voltage divider to the first circuit whenthe at least one second switch has the first state and electricallyconnecting the output of the voltage divider to the second circuit whenthe at least one second switch has the second state.
 19. The sensor ofclaim 18, wherein the voltage divider includes a programmable voltagedivider.
 20. The sensor of claim 19, wherein the output of theprogrammable voltage divider can be adjusted to vary the resistance towhich the first and second resistors are set.
 21. The sensor of claim19, wherein the programmable voltage divider includes a plurality ofresistors connected between the output of the voltage divider and areference terminal, and an output voltage of the voltage divider can bevaried based upon which of the plurality of resistors are connectedbetween the output of the voltage divider and the reference terminal.22. The sensor of claim 21, wherein the output voltage of the voltagedivider can further be varied based upon an amount of time each of theplurality of resistors are connected between the output of the voltagedivider and the reference terminal.
 23. The sensor of claim 19, whereinthe programmable voltage divider includes a digital to analog convertercircuit having an output that sets the output of the voltage divider.24. The sensor of claim 23, wherein the digital to analog convertercircuit includes: a digital to analog converter having an output thatprovides a variable amount of current; and an amplifier circuit havingan input electrically coupled to the output of the digital to analogconverter, and an output that forms the output of the analog to digitalconverter circuit and provides a variable output voltage based upon thevariable amount of current.
 25. The sensor of claim 18, furthercomprising: a first hold capacitor, electrically connected to the firstcircuit, that maintains the resistance of the first resistor when the atleast one first switch and the at least one second switch have thesecond state; and a second hold capacitor, electrically connected to thesecond circuit, that maintains the resistance of the second resistorwhen the at least one first switch and the at least one second switchhave the first state.
 26. The sensor of claim 18, wherein the at leastone second switch receives a switching signal that switches the at leastone second switch to the first state after the at least one first switchhas switched to the first state.
 27. The sensor of claim 26, wherein theswitching signal switches the at least one second switch to the secondstate after the at least one first switch has switched to the secondstate.
 28. The sensor of claim 17, wherein the voltage divider is sharedbetween the first and second circuits.
 29. The sensor of claim 17,wherein only a portion of the voltage divider is shared between thefirst and second circuits.
 30. A method for use with a pair of bridgecircuits each having a sensor leg that includes a fixed resistor and avariable resistor and a reference leg that sets a resistance of thevariable resistor, the method comprising an act of: sharing at least aportion of the reference leg between the first and second circuits tomatch the resistance of the variable resistors.
 31. The method of claim30, wherein the act of sharing includes an act of: switchably connectingthe shared portion of the reference leg to each of the pair of bridgecircuits at different times.
 32. The method of claim 30, wherein thereference leg includes a fixed portion and a variable portion, andwherein the act of sharing includes an act of: sharing the variableportion of the reference leg between the first and second circuits tomatch the resistance of the variable resistors.
 33. The method of claim30, wherein the reference leg includes a fixed portion and a variableportion, and wherein the act of sharing includes an act of: sharing boththe variable portion of the reference leg and the fixed portion of thereference leg between the first and second circuits to match theresistance of the variable resistors.
 34. A flow sensor to measure aflow rate of a fluid, comprising: a first variable resistor; a secondvariable resistor disposed downstream of the first variable resistorwhen a flow of the fluid is in a first direction; a first circuit,electrically coupled to the first variable resistor, to provide a firstsignal indicative of power provided to the first variable resistor; asecond circuit, electrically coupled to the second variable resistor, toprovide a second signal indicative of power provided to the secondvariable resistor; and a third circuit, to receive the first and secondsignals and provide an output signal indicative of a difference betweenthe first and second signals; wherein a range of the output signal whenthe flow of fluid is in the first direction is symmetric to a range ofthe output signal when the flow of the fluid is in a second directionthat is opposite to the first direction.
 35. The flow sensor of claim34, wherein the third circuit includes: a first amplifier circuit toprovide a third signal indicative of the difference between the firstand second signals; a second amplifier circuit to provide a fourthsignal indicative of a sum of the first and second signals; and aconverter circuit to receive the third signal and the fourth signal,divide the third signal by the fourth signal to provide a dividedsignal, and provide the divided signal as the output signal.
 36. Theflow sensor of claim 34, wherein the converter circuit includes anAnalog to Digital converter having a differential input to receive thethird signal and a reference input to receive the fourth signal.
 37. Aflow sensor to measure a flow rate of a fluid, comprising: a firstvariable resistor; a second variable resistor; a first circuit,electrically coupled to the first variable resistor, to provide a firstsignal indicative of power provided to the first variable resistor; asecond circuit, electrically coupled to the second variable resistor, toprovide a second signal indicative of power provided to the secondvariable resistor; a third circuit, to receive the first and secondsignals and provide an output signal indicative of a difference betweenthe first and second signals; and a power supply circuit, electricallyconnected to at least one of the first and second circuits, to provide avariable amount of power to at least one of the first and secondcircuits dependent upon the flow rate of the fluid.
 38. The flow sensorof claim 37, wherein the power supply circuit is electrically connectedto each of the first and second circuits, to provide the variable amountof power to each of the first and second circuits dependent upon theflow rate of the fluid.
 39. The flow sensor of claim 37, wherein thepower supply circuit decreases the variable amount of power provided toat least one of the first and second circuits at low flow rates andincreases the variable amount of power provided to at least one of thefirst and second circuits at high flow rates.
 40. A method of detectinga high flow condition in a flow sensor, comprising acts of: determiningan expected zero flow signal at a current operating temperature of theflow sensor; determining a threshold based upon the expected zero flowsignal; determining an actual flow signal measured by the flow sensor atthe current operating temperature of the flow sensor; comparing theactual flow signal measured by the flow sensor to the threshold; anddetermining that the high flow condition exists when the actual flowsignal exceeds the threshold.
 41. The method of claim 40, wherein theflow sensor includes an upstream circuit that provides a first outputsignal indicative of power provided to an upstream coil of the flowsensor and a downstream circuit that provides a second output signalindicative of power provided to a downstream coil of the flow sensor,wherein the act of determining the expected zero flow signal includes anact of determining a sum of the first and second output signals at azero flow rate at the current operating temperature of the flow sensor.42. The method of claim 41, wherein the act of determining the thresholdincludes an act of multiplying the expected zero flow signal by aconstant.
 43. The method of claim 42, wherein the act of determining theactual flow signal includes an act of determining a sum of the first andsecond output signals at a current flow rate at the current operatingtemperature of the flow sensor.
 44. The method of claim 43, furthercomprising an act of preventing an amount of power provided to theupstream coil and the downstream coil from increasing excessively inresponse to the act of determining that the high flow condition exists.45. The method of claim 44, wherein the act of preventing includes anact of regulating an amount of power provided to the upstream coil andthe downstream coil.
 46. The method of claim 45, wherein the act ofregulating the amount of power includes an act of regulating a voltageprovided to the upstream coil and the downstream coil.
 47. The method ofclaim 46, wherein the flow sensor provides a sensor output signal thatis based upon a difference between the first output signal and thesecond output signal, the method further comprising an act of settingthe sensor output signal to a high value in response to the act ofdetermining that the high flow condition exists.
 48. The method of claim47, wherein the high value is dependent upon a direction of a flow offluid through the flow sensor.
 49. The method of claim 40, wherein theflow sensor includes an upstream circuit that provides a first outputsignal indicative of power provided to an upstream coil of the flowsensor and a downstream circuit that provides a second output signalindicative of power provided to a downstream coil of the flow sensor,the method further comprising an act of preventing an amount of powerprovided to the upstream coil and the downstream coil from increasingexcessively in response to the act of determining that the high flowcondition exists.
 50. The method of claim 49, wherein the flow sensorprovides a sensor output signal that is based upon a difference betweenthe first output signal and the second output signal, the method furthercomprising an act of setting the sensor output signal to a high value inresponse to the act of determining that the high flow condition exists.51. The method of claim 50, wherein the high value is dependent upon adirection of a flow of fluid through the flow sensor.
 52. The method ofclaim 40, wherein the flow sensor includes an upstream circuit thatprovides a first output signal indicative of power provided to anupstream coil of the flow sensor and a downstream circuit that providesa second output signal indicative of power provided to a downstream coilof the flow sensor, wherein the flow sensor provides a sensor outputsignal that is based upon a difference between the first output signaland the second output signal, the method further comprising an act ofsetting the sensor output signal to a high value in response to the actof determining that the high flow condition exists.
 53. The method ofclaim 52, wherein the high value is dependent upon a direction of a flowof fluid through the flow sensor.
 54. The method of claim 40, whereinthe expected zero flow signal and the actual flow signal are indicativeof a total amount of power provided to the flow sensor at a zero flowrate and a current flow rate, respectively, at the current operatingtemperature of the flow sensor.
 55. The sensor of claim 17, wherein thesensor is a flow sensor that senses a flow rate of a fluid flowing in aconduit, the sensor further comprising: a power supply circuit,electrically connected to at least one of the first and second circuits,to provide a variable amount of power to at least one of the first andsecond circuits dependent upon the flow rate of the fluid.